In a data transmission method which converts information such as a voice signal into digital data, and transmits the data after conversion, an amount of information to be transmitted is not temporally fixed, but varies from time to time in general.
Accordingly, the transmission rate can be varied by dividing the transmission data into fixed length frames, and by transmitting variable bit length data in individual frames. This makes it possible for a transmitter to effectively transmit the information at a constant frame period, and to avoid needless transmission, thereby saving consumed power.
To achieve the data transmission at a variable rate, it is necessary for a receiver to acquire information about the transmission rate of each frame by some means. Conventionally, there are proposed two methods to achieve this: A first method transmits rate information of each frame as part of the frame data so that the receiver can decide the rate based on that rate information; and a second method does not transmit the rate information, but utilizes error detection codes added to the transmission data for representing the communication quality, in which case the receiver decides the rate based on the error detection codes.
On the other hand, in communications environments such as data transmission through radio channels, in which a lot of errors will occur, error correction (FEC: Forward Error Correction) of the transmission data is widely carried out to improve the quality of the transmission. As the error correcting code and error correcting decoding, a convolutional code and maximum likelihood decoding such as Viterbi decoding are known.
In the second method which does not transmit the rate information to the receiver, but has the receiver decide the rate from the error detecting code added to the transmission data to represent the communication quality, the error rate in the rate decision depends on the word length of the error detecting code, and even if the transmission error can be reduced, the decision error rate, that is, the probability of making a decision at a wrong rate that no transmission error has occurred, cannot be reduced below a certain level.
On the other hand, in the first method, in which the rate information is transmitted from the transmitter to the receiver, the receiver cannot decide the effective length of the received frames if an error has occurred during the transmission. It is difficult for the receiver to recover the transmitted data correctly even if the data portion itself is free from the error.
Therefore, an object of the present invention is to solve the foregoing problems, and to provide a data transmission method, data transmission system, and a transmitter and receiver, which can implement high quality variable rate data transmission by making a reliable rate decision in any communications environment.